Resistor with high positive temperature coefficient



March 1952 R. SCHOFER ETAL RESISTOR WITH HIGH POSITIVE TEMPERATURE COEFFICIENT Filed April 28, 1959 2 Sheets-Sheet 1 Fig.1

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March 1962 R. SCHGFER ETAL 3,0 ,5

RESISTOR WITH HIGH POSITIVE TEMPERATURE COEFFICIENT Filed April 28, 1959 2 Sheets-Sheet 2 Uite 3,027,529 RESISTOR WITH HIGH POSITIVE TEMPERA- TURE COEFFICIENT Rudolf Schiifer, Walter Heywang, and Erich Fenner, Munich, Germany, assignors to Siemens & Halske Alrtiengesellschaft Berlin and Munich, a corporation of Germany Filed Apr. 28, 1959, Ser. No. 809,478 Claims priority, application Germany Apr. 30, 1958 11 Claims. (Cl. 338-28) This invention is concerned with a resistor having a high positive temperature coefficient of resistance, which may be considered in the nature of an improvement on the resistor disclosed in the copending application Serial No. 734,818, filed May 13, 1958, now abandoned, owned by the assignee also named in the present application.

The resistor described in the above noted copending application has a positive temperature coefiicient of resistance Within the entire temperature range or, preferably, only within the upper part of the operating temperature range, and is characterized by the following features,

namely (a) the resistor consists of ferromagnetic material having a Curie temperature, above which the material loses its permanent polarization, lying at least below the upper limit of the operating temperature range, especially at or below the lower limit of the range in which the resistor shall have a positive temperature coefficient of resistance, and preferably below C.; (b) the material is made conductive by impurity centers, preferably n-co'nductive, whereby the spacing E of the donors from the line band and of the acceptors from the valence band is smaller, especially considerably smaller than half the width E of the prohibited zone between the valence band and line band; and (c) the intrinsic conductivity or another impurity conductivity of the material is low, being particularly negligibly low as compared with impurity center conductivity, at least in a part of the operating temperature range in which the resistor has a positive temperature coefficient.

it was found in the production of such resistors that the resistance value often exhibits an undesired voltage dependence. The object of the present invention is to eliminate this voltage dependence or to reduce it to a point at which it is not disturbing. The copending application Serial No. 734,818 states in this connection that the resistor body, after the contacting thereof, is to be momentarily placed on high voltage, that is, that it be formed by a strong current surge.

The present invention is based upon thoughts and investigations concerning the possible reasons for the dependence of the value of the total resistance lying between the two current leads, on the polarization and/or on the magnitude of the voltage at the terminals. It must be considered in this connection that several reasons for the voltage dependence of the total resistance value are involved in the case of resistor bodies consisting of sintered ferroelectric crystallites, namely, as a volume effect, the voltage dependence of the transistion resistance be- "tween the adjoining crystallites in the resistor body as such; secondly, border layer effects of the semiconductor, for example, the formation of blocking or barrier layers in the marginal layer of the semiconductor which are caused by impurity centers of the semi-conductor located upon or closely to the surface thereof or by contaminations, for example, due to adsorbing oxygen; and, third,

as surface border effect, a rectifying action between the resistor material which has been made conductive and the contact metal serving for the current connection, which is provided, for example, vaporized upon the semiconducting resistor material for the contacting thereof.

The various objects and features of the invention will ice appear in the course of the description which is rendered below with reference to the accompanying drawings, in which:

FIG. 1 shows an embodiment of the invention;

FIG. 2 indicates another embodiment;

FIG. 3 is a graph to illustrate an n-doped titanate and the formation of the potential wall; and

'FIGS. 4 and 5 show examples of electrically heated appliances using a semiconductor according to the invention.

The invention proposes to reduce the voltage dependence of the total resistance value of such resistors or to make it negligibly low at room temperature, by placing the contacts upon the material of the resistor (FIG. 1) substantially free of barrier or blocking layer, especially, to vaporize the contacts thereon. The material for the contacts is for this purpose selected so that it does not form a barrier or blocking layer with the resistor material; more particularly, in the case of a resistor sintered of ferroelectric crystallite particles made n-conductive, the current supply contacts will consist of a base metal, preferably aluminum or zinc or of an alloy containing at least a high proportion of one of these metals. The surface parts of the ceramic material serving for the contacting are moreover prior to or if desired incident to the contacting preferably particularly treated as compared with the interior particles of the resistor body, especially mechanically treated, for example, they are made to be well conductive by solder rubbing or by sanding or by electrical or chemical treatment applied prior to vaporizing metal thereon. The purpose of this pretreatment is to provide a clean surface free of troublesome conditions.

This may be obtained, for example, by glow treatment or by chemical reduction. The treatment may be such as to affect not only the surface but penetrating to some depth the marginal layer of the resistor material.

In order to keep as small as possible the above mentioned varistor-like volume effect caused by the transition resistances between the crystallites of the sintered resistor body, it is furthermore proposed that the resistor material be sintered at suitable high temperature for a sufiicient time so as to avoid these transition resistances as far as possible. In the event that an impermissible voltage dependence of the specific resistance cannot be avoided in the production of the resistor body, by suitable selection of the sintering conditions, the volume effect may be subsequently reduced by forming operations. For example, as explained in the copending application Serial No. 734,818, a sufficiently voltage-independent resistor may be produced by first sintering the resistor body without regard to the voltage dependence of its resistance, thereupon contacting the resistor body in the manner explained above, and thereafter passing through the resistor a strong current surge which reduces the voltage dependence of the transition resistances between the crystallites and therewith the conductivity of the resistor body to a tolerable point, by effecting in a manner welding together of the individual crystallites.

Since a reduction of the volume resistance could in connection with the above noted treatment he observed if at all-only at high field strength, while the resistance remained at lower field strength practically independent of the voltage applied, it is possible by providing suitable dimensions of the resistor body to produce for any given voltage a resistor with any desired resistance value, which is practically free of varistor elfects, by making the spacing between the two resistor contacts relatively great, therewith the field strength low, and making the crosssectional area of the resistor body correspond to the desired capacitance and the length of the current path in the resistor body.

The surface treatment is in a particularly advantageous pole of the" voltage source;

; body made of sintered ferroelectric crystallites.

. 11,12 are cleaned as provided by the invention, for ex- Y ample by electrical orchemical pretreatment. The con- :tact metal is vaporized upon these end surfaces (see dash s lines-13 and 14), such contact metal being preferably identical metal or identical alloy at the two ends. current supply terminals were connecteddirectly to these metallic layers 13, 14, the latter, in the use of the resitor, would be subjected to considerable stress and it is, therefore, advantageous to avoid such stress that would occur, for example, in soldering the resistor in place in a given apparatus.

I placed at the resistor ends with press fit,'such metal caps 7 being provided with openings 15' and 16 formed therein. ,The margins of these openings are directly soldered embodiment carried out by subjecting the surface to a glow effect. For this purpose, the semiconductor is suitably disposed in a vacuum vessel, at relatively low gas j pressure; opposite an electrode and the surface parts of mthe semiconductor body which are subsequently to be body and theelectrode. When using a direct voltage, it

is advisable to 'place'the semiconductor on the positive The resistor material is pref- 'erably subjected to a strong glow effect, for example, at :a. current density from about 10 to milliamperes per rsquare centimeter, for example, at roughly 3000 to 5000 volt, so as to liberate the surface to be metallized also T from adhering residual gas or other contaminations such :as deposited'hydrogen. The contact metal which does not form a blocking layer with the semiconductor is l thereupon applied, advantageously by vaporization in the same-vacuum vessel at further reduced gas pressure.

The contact metals to be used in the case of n-doped ferroelectric resistor materials are preferably base metals with a normal potential below that of silver, especially below that of copper, for example, Al or Zn, which are particularly suitable. The use of noble metals as contact materials is, however, not inherently excluded.

\ The current connections are preferably mechanically secured on the semiconductor body; for example, in the case of rod-shaped resistor bodies, in the form of caps 1 attached thereto by press fit or in the form of clasps em- 4 bracing the corresponding body. An example is illustrated in FIG, 1.

? Referring to FIG. 1, numeral 1 indicates the resistor Its ends If the For this purpose, metal caps 15, 1.6 are (15", 16") to the-vaporized metal layers 13, 14, thereby providing a satisfactory electrical contact between respective contact layers 13, 14 and the caps l5, 16.

Pure tin is particularly to be recommended as the solder for connecting the caps 15;16, consisting, for example, .of copper, with the vaporized layers 13, 14, since the resistor mustfrequently be heated to relatively high .temperatures of 200 C101 thereabout to bring it to its maximum specific resistance. This means, generally speaking, that the melting point of the solder used must lie considerably above the Curie point of the ferroelectric material of the resistor 1.

In order to facilitate the soldering in place of the caps and the current connections to the contact layers of the resistor 1, it is furthermore proposed to coat the vaporized base metal layers which consist, for example, of aluminum, chemically or electrochemically treated, for example, in a galvanic bath or by burning-in of silver, with a copper or especially a silver layer, to-which can be easily soldered the caps 15,16.

However, in the burning-in of silver, the alloying of the silver into the base metal layer must be 'avoidedfparticularly by the application of relatively low temperatures.

because the good contact properties of the base metal a would otherwise deteriorate.

It is in many cases, especially'in the case of a diskshaped embodiment of the resistor l, as shown in FIG. 2,

advisable to use instead of caps which are mechanically fastened to the resistor body, s'olderable material provided chemically or electrochemically upon the base metal layers and to solder the current leads l7, 13 by means of high melting solder 39, 2t), directly to the layers 35, 16 consisting of solderable material. c

In FIG. 2, numeral l-again indicates the resistor body of a-length L which is in this case disk-shaped. Its surfaces ll, 12 are pretreated according to the invention or, for example, coated with base metal 13, 14 by a. rubbing operation. It is also possible, after freshening the sur face layers ll, 12 for removal of surface conditions, to vaporize a base metal thereon, for example, aluminum 13, 14. These vaporized layers 13,-- 14- form very good barrier-free contacts with the surfaces 11, 12- of the ceramic resistor'and are provided with solderable metal layers l5, 16 which are placed in position chemically or electrochemically.

The current leads 17,-1.3 are thereupon over a wide area directly soldered to the reinforcing layers 15, 16, consisting preferably of silver, by'means of solder 19, Ztl, consisting of pure tin. The current leads 1'], 18 are for this purpose by stamping or the like provided with 1 disk-shaped enlargements 171, 181 at their ends facing the layers 15, 16.

Experiments have shown that the method according to the invention does not only avoid the appearance of blocking or barrier layers; the resistors also have a particularly low minimum valueo'f specific resistance which exhibits in the case of; barium titanate, made conductive, always values below ohms per centimeter. The rise of the resistance value depending upon the temperature 1 thereby becomes considerably steeper.

As already mentioned above, rubbing solder may be used in place of the electrical or chemical pretreatment. Solders of this kind may be, for example, indium-amalgam alloys; solder composed of 40 parts Bi, 25 parts Pb, 10 parts Sn, 10 parts Cd, preferably with an addition of 15 parts indium is suitable forthis purpose. w Such rubbing solders are however unsuitable for mass production of resistors demanding clear-cut transition resistance between the resistor body and the metal, since the corresponding process is substantiallybased upon mechanical destruction of the surface layer of the ceramic body and since such surface destruction can hardly affect the entire contact surface incident to the corresponding mechanical procedure. 7

It was found'th-at even upon using the means noted in the copending application Serial No. 734,818 or the means previously noted herein, for the-elimination of the voltage dependency, that voltage dependency is frequently prescut at sufiiciently high field strength. To eliminate this voltage dependency, the dimensions of the resistor are to be such that, up to the maximum operating voltage, the field strength in the resistor material must not exceed the value at which the resistor body remains substantially free of the varistor effect.

A further object and feature of the invention is concerned with the provision of a resistor, particularly consisting of amaterial having at high field strength a voltage dependent resistance value but being free of this varistor effect at lower field strengths, in which the particle size I of the preponderant part of the ceramic resistor body, as it appears particularly in an electron microscope picture, is about l-20 microns and, especially, the smaller the greater the field strength is to be in the operating condition of the resistor body, but in no case smaller than The particles of the ceramic resisti be presently explained, understood to mean the length of the particle in the direction of the current path in the resistor.

- The-selection,- in accordance with the invention; of the ductivity band of the barium titanate.

- equal to or smaller than this quotient.

particle size, and the use of particles of uniform size, in the production of the resistor material, provides the advantage of producing a resistor which, up to thevdesired maximum field strength, will have a resistance value which is substantially independent of voltage. This effect may be explained by the presence, in the resistor material, despite the eventually effected formation, of surface states at the particle borders, which lead to the formation of a potential wall and therewith to the varistor effects.

FIG. 3 shows in schematic manner an n-doped titanate and the formation of such a potential wall. The vertical line 31 indicates the particle border, for example, of two intersintered barium titanate bodies. References 32' and 32" represent the donor parts of two particles I and II lying closely to the lower border 33' and 33" of the con- The upper limit of the valence band of those'titanates is indicated at 34' and 34". At the particle border there is at A an acceptor part which, due to its energetically low position leads to a discharge of the neighboring donors 21, 21", such donors accordingly forming a space charge zone and tending to bend the conductivity bend upwardly. The height of this bend is indicated by 90. The height of the potential wall thus designated by q), being now within the range of theCurie temperature due to the very strong tempera- I ,ture dependence of the dielectric constant (e), is to a high degree temperature dependent. It rises, for example, in the case of barium titanate at an assumed acceptor density at the particle borders, of -10 cm. and at 'a'donator density of about 10 -i0 /cm. from about volt at 20 C. to about 1 volt at 200 C.

A low voltage dependence of the resistance value occurs in the case of such potential walls, when the potential wall is by the electrons flowing in the line band substantially not overcome by the wave-mechanical tunnel effect (E at which the resistor is operated, must not be appreciably greater, at the particle borders, even at the lowest operating temperatures, than the quotient formed by the height of the potential wall divided by the particle size d. If possible, this maximum field strength shall be Expressed dif. ferently, this means, that the voltage drop at the individual particle borders, represented by the equation (U=max. operating voltage at the resistor; L=length of current path in the resistor; d=particle size measured in the direction of the current path) shall at the wherein d is the average value (average particle diameter d of the resistor material) and go min. the height q; of

thepotential wall at the lowest operating temperature.

It might be assumed from this equation that it is merely necessary to make the particle size small as desired so as to secure the voltage independence of the resistance of. the resistor material up to any desired field strengths E This assumption is, however, wrong because it does not take into account the fact that the particles must be larger than the width x of the potential wall noted in the figure. If they'are smaller, the size q) of the potential wall, as will be realized upon refiecti on, will with otherwise identical conditions also be reduced, namely, approximately quadratic with the particle size d, and the permissible maximum field strength will again decrease. The optimum desired in accordance with the invention is at a point atwhich the average size d of the particles, measured in the direction of the length of the current path, is about equal to, the value x of the width of the potential wall. At such dimensioning of the particle size, the operating field strength E of the resistor will be at a maximum, that is, the length of the current path can be short at a given operating voltage.

These considerations also show the advantages that will result when the particle sizes in the resistor are as uniform as possible, that is, when they fluctuate in the essential part of the resistor only negligibly about the average particle size; for, strong deviations upwardly or downwardly of the average particle size signify that the maximum field strength at which the resistance value is substantially still voltage-independent, is reduced either responsive to the value (p of the potential wall becoming too small (in the case of particles which are too small) or responsive to the voltage of the individual particle borders becoming too high (in the case of particles which are too large). The optimum particle size d is present, and in accordance with the invention shall be realized as closely as possible, when it is equal to the quotient of the term density (acceptor density A) on the surface of the particles divided by the impurity density (donor density D) in the resistor material (d=A:D).

It will be apparent from the foregoing considerations that it is in the production. of the resistor essential that the particle sizes in the finished resistor, that is, the particle diameters (I measured substantially in the direction of the current flowing through the resistor, correspond to the requirements. The particle diameters transverse of the direction of flow of the operating current are of lesser importance since the phenomena taking placeat the particle borders lying approximately parallel to paths of the operating current influence the voltage dependence of the resistance value only inconsiderably or not at all.

It is accordingly possible by the determination of the particle sizes and by the use of particles of approximately the same size, to make the maximum operating voltage of the resistor high, that is, to keep the length of the current path in the resistor short; and it is for this reason advantageous to produce disk-shaped resistors to be contacted barrier-free as far as possible in the manner as explained with reference to FIG. 2. The donor density of the ceramic material ll may in this case amount to 10 impurity centers per cubic centimeter and the acceptor density at the particle borders to about 10 to 10 impurity centers per square centimeter; moreover, since the particle sizes are to fluctuate substantially only slightly about the optimum value, the particles to be used in this case will have a size from about 5 to 8 microns.

Instead of choosing an optimum particle size, it is possible to proceed in accordance with the previously noted formula so as to obtain, with a given 'particle size d, a field strength as high as possible by selecting the donor density correspondingly. In the above mentioned example, this means that, with a particle size varying from 5 to 8 microns and an acceptor density A from about 10 to 10 impurity centers per square centimeter, the donor density D in the Perowskit is to be made approximately equal to 10 impurity centers per cm. that is, to dope the Perowskit with about 10 donors per com.

The technical problem attending a relay-free temperature stabilization, for example, micro-thermostats for temperature sensitive structural parts, for example, oscillating quartz elements, or for power-stabilized heating devices to be used within a Wide voltage range (for example, immersion heaters for -220 v.) or for heating elements with regulatable terminal temperature (for example, fiat irons), may in accordance with the invention be solved in simple manner by the use of resistors as described herein.

The positive temperature coefficient of these resistors is particularly suitable for a relay-free and trouble-free automatic tcmperature stabilization. of electrically heated appliances. Among the many possible applications, the invention suggests as example a circuit arrangement comprising a temperature dependent resistor the resistance value of which increases strongly with rising temperament :for temperature control and limitation according to the invention. The'heating resistor 44 is disposed in series with the resistor 42. The spacing 45 between the resistor 42 and the plate 43 to be heated can be adjusted by-l means of the'device' 41, thereby effecting adjustment.

of the desired terminal temperature. Disk-shaped c011- figuration of the resistor 42 is in this case particularly favorable.

FIG. illustrates a heating plate 56 provided with :ture, that is, by approximately one tens power at about 5 a heating resistor formed of a plurality of parallel dis- 20 temperature rise (thermistor), such resistor being posed rod-shaped resistor rods 58. Numeral 57 indicates [either connected serially 'withthe heating resistor or a mica layer serving for insulation purposes. Screws 51. serving by itself as heating resistor. extending through a pressure plate 59 are provided for Theheating of the corresponding appliance therefore pressing the resistor rods 58 against the mica layer '57 is effected, for example, in customary manner by a heat- -on the underside of the plate 56 to be heated, thusproing coil the resistance of which changes but little with I viding for as good a heat exchange contactas possible the temperature, the current flowing through such heating so that the temperature of the'plate 56 will practically coil beingconductedthrough a resistor embodying the be equal to that of the rods 58. Numeral 50 indicates described features, which is disposed in more or less fixed a cord having a plug for connecting the device to a suitheat exchange with the device to .be heated. The resistable currentsource.

ance of such resistor will steeply rise when the tempera- Changes may be made within the scope and spirit of ture reachesthe terminal value and will thus reduce the the appended claims which define What is believed to heating cu rrent. be new and desired to have protected by Letters Patent. The terminal temperature can be varied as desired by -We claim:

variation of the heatexchange relationship, for example, 1. in a resistor having a resistor body which has at ---variation of-the spacing between the resistor and the least in the upper region of the operating temperature object to be heated. The current will immediately range a positive temperature coefiicient of the resistance I strongly increase when heat is drawn from the appliance, value, said resistor body being made of ferroelectric mauntil the switching-off temperature is reached. With terial with a Curie temperature, above which suchmaterelatively small heat Withdrawal, the idling current will rial .loses its permanent polarization, lying, at least below be correspondingly low, for example, in the case of an the upper limit'of the operating temperature range, said inactiveheating plate, it will amount to a few millimaterial being conductive by impurity centers included -amperes. The magnitude of the voltage of the supply therein, whereby the respective spacing of the donors current (1-10 v.-220 v.) has due to the steepness of the from the line band and of acceptors from thevalence resistance rise practically no effect on the terminal temhand is considerably smaller than one half of the width perature. of the prohibited zone between thevalence band andthe Theheating of the appliance may, however, be efiected line band while the intrinsic conductivity or conductivity directly by the heat developed by such resistor. The due to the presence of other impurity centers is lower ceramic body of the resistor thereby takes the place of and especially negligibly low as compared with the conthe heating coil. The prerequisite for this entirely new ductivity which is due to the presence of said first named ;kind' of heating is, that the low cold resistance of the impurity centers; a device for achieving negligibly low resistor is so high that the current consumption and voltage dependence. of; the total. resistance value, said therewith the speed of heating up of the appliance is not device comprising cur-rent lead. contacts placedharriertoo great. For example, if a current of 2 amperes is to free upon said resistor body and being made of a combe obtained at 220 v., the cold resistance of the resistor 40 men metal selected from the class of metals consisting 1 must be 110 ohms. Resistors with, for example, recof aluminum and zinc or of an alloy with a high content tangular cross section and having such cold resistance, of at least one of said common metals. can be'easily produced. They are, for example, pressed 2. A resistor according to claim 1, wherein the reagainst the underside of a plate to be heated with intel'- sistor body is made of n-conductive barium titanate the position of a thin insulating mica layer. The better the specific resistance of which is at a temperature belowthe heat exchange contact is, the. quicker will the plate as- Curie temperature of the ferroelectrical basic material sume the temperature at which the resistor will cut down l than ohm per centimeter.

the current. The current taken up by the resistor rods 3, A resistor according to claim 1, having a rodlike regulates itself automatically so that the assumed wattage esisto body, comprising a contact layer disposed-pracl the heat energy given Accordingly, the 50 tically voltage independent upon said rodlike resistor hlghfil' the Supply g The Weaker l e e Current. body, and a mechanically strong metal contact part which In the corresponding Circuit arrangement, the Setting is metallically connected with saidcontact layer. of the desired terminal temperature can be effected by 4, A resistor according to claim 1 wherein i the setting of the heat exchange transition of the resistor rent lead contacts are reinforced by a layer of silver.

with respect to the body the temperature of which is to 5. A resistor according to claim 1, comprising current- J regulated Accordingly, is temperature g lead means and contact layers which are mechanically P tron, the amount of heat passing from the resistor to the and electrically firmly interconnected by solderconsistappliance is adjustable at the amount of heat generated .1 f pure in the melting temperature of which l e therein, that is, the Spacing between the resistor considerably above the Curie point of the ferroelectric example, the Plate t0 be s Variable Since basic material of the crystal particles of the resistor body, requiredfeglllatioll Operations are effected i steady particularly above the temperature at which the'resistor manner, y do not entail disturbing high t q y material assumes in operation its maximumresistance effects and the corresponding appliances require no in- 1 terference protection. Thermal overloading which limits 6, A esi tor having a resistor body which has at least .the life of customary heating elements is due to the autoi h upper region f h operatingv temperature range matic temperature limitation Practically eXchldeda positive temperature coefficient of the resistance value, Explanations as to the use of a resistor according to said resistor body being made of ferroelectric material the inventionwill now be supplied with reference to with a Curie'temperature, above which such material embodiments illustrated in FIGS. 4 and 5. loses its permanent polarization, lying at least below the 1 *FIG. 4shows a flat iron comprising a circuit arrangeupper limit of the operating temperature range, said material. being conductive by impurity centers included 7 therein, whereby. the. respective spacing of the donors band is considerably smaller than one half of-the width ot the prohibited zone between the valence baud'and the from the line band and of acceptors from'the valence line band while the intrinsic conductivity or conductivity due to the presence of other impurity centers is lower impurity centers, wherein the dimensions, with respect to length of current path and cross-sectional area thereof, are selected so that the field strength appearing in said resistor body remains so low that voltage dependence which is unavoidable in high resistance condition thereof cannot become effective.

7. A resistor having a resistor body which has at least in the upper region of the operating temperature range a positive temperature coeflicient of the resistance value, said resistor body being made of ferroelectric material with a Curie temperature, above which such material loses its permanent polarization, lying at least below the upper limit of the operating temperature range, said material being conductive by impurity centers included therein, whereby the respective spacing of the donors from the line band and of acceptors from the valence band is considerably smaller than one half of the width of the prohibited zone between the valence band and the line band while the intrinsic conductivity or conductivity due to the presence of other impurity centers is lower and especially negligible low as compared with the conductivity which is due to the presence of said first named impurity centers, wherein the particle size (d) of the preponderant part of the resistor body is approximately equal to the quotient from surface term density (A) to the impurity center density (D), amounting to 8. A resistor having a resistor body which has at least in the upper region of the operating temperature range a positive temperature coefficient of the resistance value, said resistor body being made of ferroelectric material with a Curie temperature, above which such material loses its permanent polarization, lying at least below the upper limit of the operating temperature range, said material being conductive by impurity centers included therein, whereby the respective spacing of the donors from the line band and of acceptors from the valence band is considerably smaller than one half of the width of the prohibited zone between the valence band and the line band 'while the intrinsic conductivity or conductivity due to the presence of other impurity centers is lower and especially negligibly low as compared with the conductivity which is due to the presence of said first named impurity centers, wherein the particle size of the preponderant part of the resistor body amounts to about 1 micron to 20 microns.

9. A resistor having a resistor body which has at least in the upper region of the operating temperature range a positive temperature coefiicient of the resistance value, said resistor body being made of ferroelectric material with a Curie temperature, above which such material loses its permanent polarization, lying at least below the upper limit of the operating temperature range, said material being conductive by impurity centers included therein, whereby the respective spacing of the donors from the line band and of acceptors from the valence band is considerably smaller than one half of the width of the prohibited zone between the valence band and the line band while the intrinsic conductivity or conductivity due to the presence of other impurity centers is lower and especially negligibly low as compared with the conductivity which is due to the presence of said first named impurity centers, wherein the surface of the resistor body is for barrier-free contacting thereof cleaned by the application of a pretreatment selected from the class of treatments consisting of electrical treatment, especially a glow discharge, and chemical treatment.

10. A resistor according to claim 9, wherein said resistor body is positively poled for the application of the glow discharge.

11. A resistor according to claim 9, wherein the glow discharge is eifected by the use of alternating voltage.

References Cited in the file of this patent UNITED STATES PATENTS 2,501,322 Ferguson et al Mar. 21, 1950 2,710,329 Herterick June 7, 1955 2,819,373 Allman Jan. 7, 1958 2,864,713 Lewis Dec. 16, 1958 2,870,307 Milliken et a1. Jan. 20, 1959 2,911,370 Kulcsar Nov. 3, 1959 

1. IN A RESISTOR HAVING A RESISTOR BODY WHICH HAS AT LEAST IN THE UPPER REGION OF THE OPERATING TEMPERATURE RANGE A POSITIVE TEMPERATURE COEFFICIENT OF THE RESISTANCE VALUE, SAID RESISTOR BODY BEING MADE OF FERROELECTRIC MATERIAL WITH A CURIE TEMPERATURE, ABOVE WHICH SUCH MATERIAL LOSES ITS PERMANENT POLARIZATION, LYING AT LEAST BELOW THE UPPER LIMIT OF THE OPERATING TEMPERATURE RANGE, SAID MATERIAL BEING CONDUCTIVE BY IMPURITY CENTERS INCLUDED THEREIN, WHEREBY THE RESPECTIVE SPACING OF THE DONORS FROM THE LINE BAND AND OF ACCEPTORS FROM THE VALENCE BAND IS CONSIDERABLY SMALLER THAN ONE HALF OF THE WIDTH OF THE PROHIBITED ZONE BETWEEN THE VALENCE BAND AND THE LINE BAND WHILE THE INTRINSIC CONDUCTIVITY OR CONDUCTIVITY DUE TO THE PRESENCE OF OTHER IMPURITY CENTERS IS LOWER AND ESPECIALLY NEGLIGIBLY LOW AS COMPARED WITH THE CONDUCTIVITY WHICH IS DUE TO THE PRESENCE OF SAID FIRST NAMED IMPURITY CENTERS; A DEVICE FOR ACHIEVING NEGLIGIBLY LOW VOLTAGE DEPENDENCE OF THE TOTAL RESISTANCE VALUE, SAID DEVICE COMPRISING CURRENT LEAD CONTACTS PLACED BARRIERFREE UPON SAID RESISTOR BODY AND BEING MADE OF A COMMON METAL SELECTED FROM THE CLASS OF METALS CONSISTING OF ALUMINUM AND ZINC OR OF AN ALLOY WITH A HIGH CONTENT OF AT LEAST ONE OF SAID COMMON METALS. 